The present invention relates to a method and apparatus for controlling the discharge or continuous bleed-off of water in recirculated systems or circuits, comprised in cooling towers and evaporative coolers used for mechanical refrigeration.
The main objective and purpose of this invention is to obtain a more accurate and reliable control of the discharge or bleed-off of the cooling water than those obtained by means for conventional bleeding, thus by this new way of control there shall be no need of permanent personal attendance and at the same time, avoiding waste or pilferage of water consumption.
It is well known that in all evaporative cooling processes, such as in a water cooling tower and in evaporative condensers, used for mechanical refrigeration, it is unavoidable to provoke the concentration of the solids contained in the recirculated water of the cooling circuit, which, in general, comprise of a plurality of water spray nozzles supplied by the water piped from the discharge of a water recirculating pump, which is collected in a water basin, which has means to replenish water via a valve and has a drain out pipe.
The concentration of solids in the water occurs, except in rare exemptions, because the waters of the public grid of those coming from wells, contain minerals in the form of carbonates, sulfates, etc., and as in the evaporative cooling process a part of the mass of water to be cooled is lost, by evaporation, those minerals contained in the evaporated fraction shall be retained in the rest of the mass of water increasing permanently the concentration of the solids.
This implies that to hold the system on a steady rate it shall be necessary to make-up or replenish the water lost by evaporation, incorporating a new quantity of water which brings its own content of minerals.
In view of this, it's easy to understand that after a certain time in operation, the concentration of solid minerals in the recirculating mass of water will reach extremely high values which shall force termination of operation of the equipment supposed to be cooled.
Some of the inconveniences derived from the excessive concentration of calcium carbonates and other chemical compounds (also known as "hardness"), as as follows:
a) scale build-up on the heat transfer surfaces,
b) greater abrasion and wear out of the seals, packings and rotors of the recirculating pumps,
c) stoppage or block-up of tubing and piping, of filters, and equipment being served, with danger of stopping all water circulation.
In relation to the calcium carbonate scales mentioned in (a), it is common knowledge of their effect as thermal insulators; thus diminishing the heat transmission and the overall thermal efficiency of the equipment.
In the United States it is common practice not to allow the recirculating water to concentrate any higher than 170 ppm, following the recommendations by ASHRAE (American Soc. Heating Refrig. and Air Conditioning Eng.) for water used in cooling towers and evaporative condensers.
The time it will take to reach these concentrations will depend entirely on the initial hardness of the make-up water.
To hold the concentration within the established limits, it shall be necessary to obtain a continuous dilution of the recirculated water. For a better understanding of the mechanics of the dilution, the following example should be of help:
The make-up for a cooling tower contains 100 ppm of Ca CO.sub.3 ; the recirculated water should not contain any higher than 180 ppm; which is the quantity of water make-up required for each lb. of water lost by evaporation:
______________________________________ where, P1 = water lost by evaperation (lbs.) P2 = excess water required to control concentration (lbs.) P3 = total make-up water (lbs.) where, P1 = 1 lb., then, P3 = P1 + P2 therefore, P3 .times. 100 ppm = (P1 .times. 0 ppm) + (P2 .times. 180 ppm) ##STR1## P3 = 1 + 1.25 = 2.25 lbs. ______________________________________
Therefore, if of the 2.25 lbs. make-up which enter the recirculating circuit, 1 (one) lb. is lost by evaporation, the excess of 1.25 lb. must be eliminated by some other means, in a continuous manner, to hold the process in a steady state.
In practice, when the hardness of the make-up water is close or higher that the established limit of concentration, the problem is solved via external chemical treatment or water softening or via internal treatment with additives fed into the recirculating waters.
Therefore, excepting the case when soft water, with zero hardness is used for make-up, there is always a need to provoke the discharge of a fraction of the recirculated water to hold the dilution under control and/or for eliminating the solid matters and residual muds from the chemical treatments and dust precipitated from the air going through the tower.
There are normally two ways to attain the continuous discharge or bleed-off in cooling towers and evaporative condensers:
a) by overflow of the water basin level, PA1 b) by diverting to the drain part of the water flowing through the recirculation piping.
The first of the methods mentioned above has been depicted in FIG. 3, shown on a cooling tower, which normally comprises a tube (1) which receives the incoming hot water, with a series of nozzles (2) for spraying water over a heat exchanging surface (3) to attain a heat transfer of heat from the water to a mass of air induced by a fan (4).
The water is collected in a basin (5), which has a pipe (6) for make-up water through a valve 7, controlled by float 8, and a conduit 9 for removing the cooled water by means of pump 10 which delivers to pipe 11 to the recirculating circuit where the cycle is completed returning the heated water back to the nozzles 2. The basin 5 also has a drain pipe 12 into which the overflow pipe 13 is connected to cause the continuous bleed-off of the circuit.
The method just described, for continuous bleed-off, is not advisable because of several reasons, the main one because the water shall continue flowing out of the basin through 13 even after the pump has been stopped, which means a waste of water; another reason is the lack of a precise control of the amount drained on account of the oscillations on the surface of the water in the basin, since as the velocity of discharge is a function of ##EQU1## these fractional differences of level can represent large fluctuations of water drained out unnecessarily.
The second method mentioned above, that is, extracting water from the recirculating piping, has been represented in FIG. 4 for a cooling tower similar to the one shown in FIG. 3, and in FIG. 5 for an evaporative condenser.
In the case of FIG. 4, the pipe (1), hot water inlet, is linked with drain 12, via a valve 15 through pipe 14; valve 15 controls the rate of bleed-off of the recirculated system and it's held at an almost constant pressure. In this example, pump 10 delivers through outlet 11 the cold water from the basin 5, when the pump is stopped so shall the bleed-off.
In the case of FIG. 5, which represents an evaporative condenser, the discharge or bleed-off is also controlled by valve 15', installed on pipe 14', which connects pipe 11' coming from pump 10 with the drain pipe 12; here again the valve 15' operates under the hydrostatic pressure as in FIG. 2.
The arrangement described as the second method is perfectly acceptable in practice, as long as the amount of bleed-off is of great magnitude (gpm), otherwise the opening of the valve will be so small that any minor particle or dirt or debris circulating with the water can plug up the flow.
It's opportune to mention that in most large installations there is trained personnel, and sometimes laboratories, in charge of controlling the quality of the make-up water as well as controlling the amount of bleed off. This means that where real help is needed is in small and medium size installations and particularly if the control of the water hardness can be done with a minimum of personal attendance.
The category of small and medium size installation of cooling towers and evaporative condensers falls between the ranges of 100,000 up to 4 million BTU per hour.
In these types of thermal equipment, the heat exchanging takes place with saturated air at about 95 degrees Fahrenheit, at this temperature the latent heat of vaporization is 1039 BTU per lb.
Table 1 shows the quantity of water lost by evaporation for several heat loads and the five columns on the right the corresponding bleed-offs, in GPH (gal. per hour) required to hold a steady concentration of 180 ppm, without the addition of chemicals, using different concentrations of ppm in the make-up water.
TABLE 1 __________________________________________________________________________ Thermal NET Rate of Bleed-off required - Gal/Hour Load EVAPORATION Hardness Hardness Hardness Hardness Hardness BTUH Loss Gal/Hour 50 ppm 75 ppm 100 ppm 125 ppm 150 ppm __________________________________________________________________________ 100,000 11.5 4.4 8.2 14.4 26.1 57.5 250,000 28.7 11.0 20.5 35.9 65.2 143.5 500,000 57.5 22.1 41.1 71.9 130.7 287.5 1,000,000 115.0 44.3 82.2 143.8 261.4 575.0 2,000,000 230.0 88.5 164.3 287.5 522.7 1150.0 4,000,000 460.0 177.0 328.7 575.0 1045.4 2300.0 __________________________________________________________________________
In cooling towers and condensers as those illustrated in FIG. 3 through 5, the average head in the recirculated circuit is 5.00 meters water column. The velocity of discharge through an orifice is ##EQU2## and the size of the orifice shall be a function of the flow in cubic meters per second to be bleed-off.
For reasons to be explained further on, the Table 2 has been prepared with the orifice sizes required for the continuous bleed-off for the GPH indicated in Table 1. A value of c=0.7 has been assumed to calculate all the orifices.
TABLE 2 ______________________________________ THER- MAL Diameter of the orifices for Bleed-off (inches) Load Hardness Hardness Hardness Hardness Hardness BTUH 50 ppm 75 ppm 100 ppm 125 ppm 150 ppm ______________________________________ 100,000 0.0364 0.0496 0.0658 0.0886 0.1314 250,000 0.0575 0.0784 0.1039 0.1400 0.2076 500,000 0.0815 0.1111 0.1473 0.1984 0.2942 1,000,000 0.1154 0.1572 0.2086 0.2809 0.4166 2,000,000 0.1631 0.2222 0.2948 0.3970 0.5888 4,000,000 0.2306 0.3142 0.4169 0.5613 0.8324 ______________________________________
As mentioned earlier, the flow, for the bleed-off, is controlled by means of a valve. It's customary to use globe or needle valves for this purpose, therefore the amount of water flow will be defined by the annular opening formed between the valve seat and the conical plunger.
Assuming a cooling tower were using a 1/2" globe valve and it were necessary to adjust the bleed-off to drain 143.8 GpH (see Table 1 for 1 MM BTUH) with make-up water with 100 ppm), then the free area of the annular section must be equivalent to the cross-section of an orifice of 0.2086" diameter; assuming the diameter of the valve seat were 0.5000", then the conical plunger would have to be introduced until the clearance was 0.0228". It's obvious that even minute particles of dirt will be sufficient to obstruct the pass of the water and consequently provoke an alteration of the GpH blow-down original planned.
In instances when there is a shortage of make-up water or when the hardness is higher than 125 or 150 ppm, it shall be necessary to use chemical products that will modify (increase) the solubility of calcium carbonates in the water; this way you'll lessen the scaling formations on the heat transfer surfaces. For example, holding a concentration of 2.5 ppm of polyphosfate in the recirculating water, for the same load of the above example (1 MM BTUH), with make-up water with 100 ppm, the continuous blow down shall be 64 GPH instead of the 143.8 GPH required with no chemical treatment.
It is frequent to find water which contains 300 ppm and even 600 ppm of hardness; assuming the same load of 1 MM BTUH, with make-up water with 300 ppm, holding the concentration of polyphosfate in 4.5 ppm, the blow down shall be 181 GPH.
The examples mentioned above are proof of how difficult it is to control properly the continuous blow down in a recirculating circuit serving a small or medium size installation, such as cooling towers and evaporative condensers.
The improvements attained with the present invention will allow a more accurate and reliable way of controlling the continuous blow down or bleed-off than the methods in current use, particularly for minimum flows of water.
The here proposed improvements warrant an almost non-clogging condition, a very accurate flow control, and with virtually no attendance required; the cost of the apparatus is very low and it is adaptable to all types of cooling tower and evaporative condensers.